Deck Calculator

What this deck calculator does

This calculator works out the decking and joist quantities for a rectangular deck — total square footage, board count, joist count, linear feet of decking and joist lumber, and a national-average cost range for the framing lumber. It supports the three most common board widths (5.5”, 5.25”, 3.5”) and three joist-spacing standards (12”, 16”, 24”). A 10 % waste factor is applied by default. All math runs on your device — your deck plans never leave the page.

How to use the deck calculator

1. Enter deck length (the direction the boards run) and deck width (perpendicular). Most decks are quoted as length × width; if yours is irregular, run each rectangular section separately.
2. Pick a board width. 5.5” is the standard for 5/4×6 pressure-treated pine (the most common residential choice). 5.25” matches the dominant composite boards (Trex, TimberTech). 3.5” is for 2×4 stringer-style decks (less common today).
3. Pick a joist spacing. 16” is the long-standing US code default for residential decks; 12” is now required by most major composite manufacturers; 24” appears only on framing below the joists, not for decking surfaces.
4. Adjust the waste slider. 10 % is the trade default; bump higher for angled or picture-framed layouts.
5. Read the headline numbers, then check the Linear feet card — that’s what you’ll quote at the lumber yard.

Composite vs pressure-treated lumber

Two questions decide your deck’s material: how long should it last, and how much maintenance are you actually going to do?

Pressure-treated pine is the workhorse. Cost runs about $1.50–3 per linear foot for 5/4×6 boards and roughly $15–18 per linear foot installed. Expected life is 10–15 years, but only with annual cleaning and a stain or sealer refresh every 2–3 years. Skip that refresh and the lifespan drops sharply — surface checks, splits, and grey weathering arrive within 5 years. Most homeowners overestimate how religiously they’ll maintain a wood deck.

Composite decking (capstock-protected, made of recycled wood fibre and HDPE plastic) costs $4–8 per linear foot for the boards and $25–35 per linear foot installed. Expected life is 25–30 years with the only maintenance being an occasional rinse and an annual sweep of debris from board gaps. The premium brands (Trex Transcend, TimberTech AZEK) carry 25-year fade and stain warranties that pay out a meaningful percentage of replacement cost if anything goes wrong.

PVC decking (all-plastic, no wood fibre) is the highest tier. Cost $8–14 per linear foot; lifespan 30+ years; the lightest and coolest underfoot of any option.

Over a 25-year ownership window, composite usually beats PT on lifetime cost — but only if you’d actually have done the stain refresh every two years. If you would not, composite was a financial no-brainer from day one.

Joist spacing matters more than you’d think

The single most common deck-building mistake is using 16” joist spacing under composite boards because “that’s what the old deck had.” Major composite brands (Trex, TimberTech, Fiberon) specify 12” on-centre for residential walking surfaces and 8” for stairs or 45° patterns. At 16” the boards visibly sag between joists in summer heat — you can feel the wave underfoot. The warranty void is also non-trivial: composite warranties are part of why the boards cost twice what wood does, and using non-spec spacing voids most of them.

16” remains correct for pressure-treated pine and most hardwood decking — that’s what US prescriptive code (IRC R507) was written around. 24” appears only as the spacing of framing below the joists (the beam-to-beam spacing), never as decking spacing.

Framing rule of thumb

For a standalone (non-attached) deck of typical residential size:

• Footings: One concrete pier per post. 12” sonotubes 36–48” deep for most US climate zones.
• Posts: Every 8 feet along each beam line. 6×6 PT is the modern standard; 4×4 PT is acceptable only for decks under 4 ft off the ground in some jurisdictions.
• Beams: Run perpendicular to the joists, supported by posts at most 12 feet apart. Doubled 2×8 or 2×10 PT is typical.
• Joists: 2×8 PT at 16” o.c. for wood decking, 12” o.c. for composite. Maximum unsupported span follows IRC R507.6 (about 10 ft for 2×8 at 16”). The calculator gives joist count; verify span capacity against your local code.

What’s not in the calculator

The calculator covers the two big-ticket material lines — decking boards and joists. Don’t forget to budget for footings, posts, beams, railings, stair stringers, hidden fasteners, joist hangers, post bases, and ledger bolts. On a typical 16 × 12 ft attached deck with one short stair and pressure-treated railing, those line items add roughly $1,200–$3,000 on top of the lumber numbers shown here. For pure framing material, a working rule is that hardware runs 8–12 % of decking-plus-joist cost.

Why 10 % waste

Deck waste is consistent and predictable. Cut-offs at the ends of each board, the off-cut when a board doesn’t reach a corner, and the occasional bow or twist that has to be discarded all add up. 10 % is the trade default and covers a straight rectangular deck. Bump to 12–15 % for picture-framed designs (where the perimeter board runs perpendicular to the field), 15–20 % for diagonal layouts (every cut is a 45° miter and the off-cuts are unusable).

Privacy

This calculator does its arithmetic in JavaScript on your device. There is no fetch call, no analytics on the values you enter, no server-side logging. The page works the same way offline once loaded.